When data such as images or audio is transmitted and received via a network, the amount of data transmitted per unit of time (transfer rate) is in some cases changed in accordance with the quality of the data. For example, in a remote monitoring system using a monitoring camera, the quality of image data transmitted from a monitoring camera to a monitoring terminal is sometimes set to be low when no abnormality has occurred. In such a case, an operator who has found abnormality switches the quality of image data transmitted from the monitoring camera to the terminal device to high quality. Image data transmitted to the monitoring terminal after the execution of the switching process is better than that of images before the execution of the switching process, and accordingly the transfer rate of transmitted data becomes higher than that before the switching. In some cases, communication routes are also changed on an as-needed basis in accordance with the changing of the transfer rate of transmitted data.
As a related art, a system is known in which low quality image data of a monitoring subject is transmitted to a monitoring device via a low speed network in normal situations. In this system, when abnormality has been detected, high quality image data of the monitoring subject and abnormality report information are transmitted to the monitoring device via a high speed network. A system is also known that includes a plurality of input devices that compress and transmit images picked up by a monitoring camera, a plurality of output devices that expand the compressed images obtained from the input devices via the network and output the images to a screen, and a device that manages the correspondence between the input devices and the output devices.
Documents such as for example Japanese Laid-open Patent Publication No. 2003-259343, Japanese Laid-open Patent Publication No. 2000-69455, etc. are known.
In a system in which transfer rates change in accordance with the changing of the quality of transmitted image data, a plurality of lines with different transfer rates have to be kept ready to be used between the transmission source and the transmission destination of image data in order to switch routes in accordance with transfer rates. This makes communication efficiency lower than a case where one route connects the transmission source and the transmission destination of image data. Although examples of images from monitoring cameras are used in the above descriptions, similar problems may occur when the quality of transmitted or received image data is switched even in a case where the data is image data, audio data, etc., i.e., data other than image data obtained by a monitoring camera.